In drilling oil wells, it is desirable to log the different earth formations, well temperature, bore hole deviation, etc. as the well is being drilled. To do so, various sensing or recording instruments are placed in the drill string, generally near the drill bit, to log this data. It is also desirable to transmit this data to the surface while the well is being drilled. The transmission of this data to the surface during drilling is a difficult process because of numerous transmission problems that must be overcome.
The most successful means of transmitting these signals to the surface presently involves the encoding of data into sequences of pressure pulses that propagate up the circulating drilling fluid medium, the pulses generally being created by valve means either momentarily restricting the flow of drilling fluid through the drill stem (providing a "positive" pressure pulse up toward the surface) or momentarily bypassing some of the flow of drilling fluid from the drill stem into the annulus between the drill string and the borehole (thus providing a "negative" pressure pulse toward the surface). The pressure pulses in turn travel through the drilling fluid to the surface where they are received by a recording instrument.
Numerous problems exist with the transmission of pressure pulses through the drilling fluid including the pulsations transmitted to the drilling mud by the drilling fluid-pump. Even with a properly installed and pressurized accumulator or desurger, the pressure surges developed by the mud pump are often of much greater amplitude than the amplitude of the data of interest and effectively mask the data signals.
One technique for transmitting data from a downhole sensor to the surface that uses pressure pulses involves converting the analog signal from the sensor to a digital signal and using the bits of the digital signal to control the opening and closing of the flow restricting valve in the flow path of the drilling fluid. In Scherbatskoy, U.S. Pat. No. 5,182,730, each transition of the voltage defining the digital value from zero volts to a relatively high voltage and back to zero defines the complete cycling of the valve. See, for example, FIG. 5A in Scherbatskoy. Such a system may reduce the interference of noise and other interfering signals but provides a very slow rate of data transmission. Further, the limit has apparently been reached in data transmission rate using available pulse technology.
Thus, there also remains a need for a data communications system for carrying data from downhole to the surface that reduces the sensitivity of the system to noise and pressure surges of the mud pump and increases the rate of data transmission.
The use of a valve to either restrict or bypass some of the fluid in the drill string into the annulus suffers other drawbacks as well. The operation of such a valve requires a source of power, commonly a battery, a set of batteries, or a turbine in the pipe segment that includes the valve and control circuitry. Since positive pulse poppet valves physically bring the "head on" mud column to a stop, they consume significant power. Such a valve typically requires 1/2 to 3/4 horsepower and draws a relatively high surge current to open or shut the valve. Also, such a valve suffers from well known erosion damage mechanisms operating in the mud environment. Thus, there remains a need for a communications system for carrying a data signal representing a parameter measured downhole that requires less power and is mechanically robust.
Known systems use "phase shift keying" ("PSK") to transmit its 0's and 1's. In known PSK, an information-bearing wave is produced by a rotating, phase-shifting siren valve and this wave is sent uphole. No phase shift signifies a logic "1" and a 180.degree. phase shift signifies a logic "0". Comparison to a reference carrier wave allows the signal to be decoded via standard demodulation schemes.
However, the implementation of PSK in the downhole environment is difficult. Rotations corresponding to a 12 Hz carrier were obtained by using a low torque electric motor in known systems; phase shifts were produced by slowing the motor down. The complete bit recognition process required a number of cycles for wave identification plus additional cycles for phase changing.
Changing phase is also complicated by timing problems. MWD-PSK ideally measures phase changes to the pressure signal, which does not necessarily correspond to specific rotor versus stator closure angles. The effects of rpm, gpm, and siren geometry on the pressure wave is still not well defined. Even if a position transducer could be used downhole, the mechanics can be very complicated.
The use of a low torque downstream rotor (see Chin et al., U.S. Pat. No. 4,785,300) in conjunction with a hydraulic motor cuts the total number of wave cycles required for phase-shifting. This is so because the hydraulic motor instantaneously effects the phase shift; fewer cycles are now needed for carrier wave identification. This has the effect of increasing data rate for the same carrier wave frequency.
In communications engineering, a reference constant phase carrier wave is always available for comparison, thus facilitating signal extraction. Because in MWD there is no reference wave uphole, computation intensive surface signal processing extracts the reference wave from the transmitted signal itself by signal processing the received signal.
At higher frequency carriers, PSK becomes increasingly unreliable in terms of bit errors. For one, rapid phase shifts introduce high frequency wave effects that are easily damped. Thus, phase-shifting using fast hydraulics in order to increase data transmission rate is not entirely beneficial; the rapidity smears the phase transition.
Second, higher frequency waves tend to be slightly dispersive (that is, the signaling speed varies with frequency), introducing some difficulties. But reflection problems, due to shorter wavelengths, become most crucial. Since operational constraints limit most MWD systems to at most two pressure transducers at the surface, more than likely at non-ideal locations, the problem of extracting a phase-shifted signal in the presence of reflected ones is difficult.
Taken together, these problems suggest that known systems have reached their optimum data rate using existing siren/PSK/hydraulics technology.